It's not bad to think about cleaning after experimenting and that depends on what you want to do.

If you want to dry distill plant matter for example and manually clean the snout of your alembic afterwards, I recommend Zen Buddhism classes. That or Propan-2-ol or concentrated sulfuric acid. Or the best: all three of the above.

I suggest writing them a kind eMail and asking how long it will take and/or if they can reserve you one. I am quite sure they will understand your english mail.

To be honest, I was foreseeing that this could happen and that they increase prices if all the alchemists start to order their alembics there. They are a very small/niche online store after all.

And their alembics are "custom made" or "mundgeblasen" (means: blown with the mouth") by a glass blower using old images/pictures of real medieval alembics as "blueprints". Difference: They are using borosilicate glass.

Don't forget to ask how much is the customs duty! You may really have better luck with that 70$ Alembic head you mentioned before and was recommended by JDP too.

Can you experienced practitioners here give me some advice on how to find the ideal size for this kind of device, please? Would it generally be advisable to acquire as big as possible an Alembic? (Especially if you were to get only one.)

Are you suggesting the butter of antimony would pass through to the receiver
or would it be collected in the alembic head ?

Substances like alcohol are wholly liquid and pass over into the alembic head, side-arm and receiver as such. It is therefore better to use alembic heads that have "channels/ridges", where the liquid partly collects and then can just be poured out. Substances like "butter" of antimony partly pass over as liquid, while other parts condense and solidify on the alembic head, side-arm and receiver. It is easier to recover these solid parts with an alembic head designed like the $70 one, which doesn't have "channels/ridges" but instead is a plain balloon-shape with a side-arm.

Depends on what you want to do (also see JDP's post right above). Are you thinking about buying the small version at Lederkram? I don't know it's exact size, but for me it would be too small. The cooling effect is smaller too..

Remember if you want to distill alcohol, even the big alembic alone won't be enough to cool it adequately unless you use a very mild heat and have a lot more time. I even used to build a DIY condenser with a silicon hose and "tin" can and attached it to the snout to not lose most of the volatile parts. It's even more a problem if you distill water alone I experienced.
A more modern approach with a suiteable condenser (not the Liebig version - less effective than others) will give better results.